As is known, energy may be stored in the form of compressed air in underground reservoirs comprising caverns or wells. Air compressors are used to fill such an underground reservoir, ideally during off peak times when the price of electricity is at its lowest (i.e. at night). The compressed air is then converted back into electricity and used during peak times when the price of electricity is at its highest. The conversion back to electricity is with a pressure engine such as turbine or piston engine in a known fashion.
One problem with the above compressed air energy storage and release systems is that as the compressed air is expanded in the pressure engine it cools rapidly on expansion. Rapid cooling of the machinery results in 1) enbrittlement of metal, rubber or plastic components, and 2) degradation of lubricants leading, both leading to excess wear, increased maintenance costs, and unexpected failure. To prevent the pressure engines from rapid cooling the air is expanded and heated before it reaches the pressure engines. The requirement for heating greatly reduces the efficiency of the compressed air storage cycle.
There have been various attempts to improve the efficiency of CAESR systems. For example, U.S. Pat. No. 4,281,256 to Ahrens for a “compressed air energy storage system” discloses a compressed air energy storage system in which an internal combustion reciprocating engine is operable as a compressor during slack demand periods utilizing excess power from a power grid to charge air into an air storage reservoir and as an expander during peak demand periods to feed power into the power grid. As an expander, the internal combustion engine utilizes a mixture of air obtained from the air storage reservoir and combustible fuel, which mixture is burned and expanded in the engine. One problem with this approach is that it requires spending a combustible fuel, and entails generation of pollution which is harmful to the environment.
As another example, U.S. Pat. No. 7,086,231 issued to Pinkerton entitled “Thermal and Compressed Air Storage System”, discloses a system which utilizes an exhaustless heater, such as a thermal storage unit, to heat compressed air prior to it entering a turbine, which powers an electrical generator. The exhaustless heater is described as being any type of heater which does not produce a waste product (e.g., a noxious or toxic emission). However, the Pinkerton system suffers from the same inefficiencies as the Jacoby system in that it requires spending energy to heat the cold compressed air. Depending on what energy source is used to power the heater, the Pinkerton system may also generate of pollution which is harmful to the environment, even if it is not noxious or toxic.
Another attempt to increase efficiency of CAESR systems is disclosed in U.S. Pat. No. 4,124,805 to Jacoby entitled “Pollution-Free Power Generating and Peak Power Load Shaving System”. Jacoby discloses a method and means whereby during periods of low load demands upon a conventional type electric power generating plant, the excess power then available is employed (at low cost to the system) to pump low temperature ambient air at relatively low pressure into a subterranean cavity in a salt deposit which is in thermal communication via an interconnecting spire or dome of salt with a geological “mother bed” occurring at such depths below the earth's surface as to constitute a constant height heat source. The air conduit system is intermittently closed, whereupon the heat intake from the earth's center causes significant storage of heat energy in the entrapped air and substantial increases of the pressure under which it is entrapped. When load requirements upon the generating plant are high, the high pressure/temperature air supply developed within the subterranean cavity is released to operate any suitably responsive turbine so as to contribute to supply of the higher power demand. A problem with Jacoby is that it requires locating a subterranean salt dome and building a generating plant located in proximity thereto. Furthermore, Jacoby requires establishing the cavity by solution mining techniques in a salt deposit which is in thermal communication via an interconnecting spire or dome of salt with a geological “mother bed” occurring at such depths below the earth's surface as to constitute a constant high heat source, which is a difficult, and expensive procedure.
U.S. Pat. App. Pub. No. 2007/0006586 published in the name of Hoffman entitled “Serving End Use Customers with Onsite Compressed Air Energy Storage Systems”, discloses a system for storing compressed air without the use of combustion in an underground void (such as a cave or mine), a below ground tank, or an above ground tank. Hoffman suggests increasing the output of the system by using solar power to heat the cold air that would enter the expander. Other sources of additional heating of the air are mentioned as well, including waste heat, geothermal and any other source heat available on the site. Although considered environmentally friendly, using solar power to power the cold air heaters is problematic in that the equipment necessary is expensive to acquire and install, and requires constant maintenance. The batteries required by the solar power system are not environmentally friendly and need to be replaced at regular intervals measured in years. Furthermore, some locations do not reliably receive sufficient sunlight to permit solar power as the sole source of power, and in most cases backup power will need to be built into the system, resulting in additional costs and inefficiencies. Using source heat that is available on the site is also problematic in that it severely limits possible locations where the system may be built.
Attempts by others are described in the following U.S. patents, each exhibiting their own problems: U.S. Pat. No. 4,150,547 (Hobson), U.S. Pat. No. 4,765,142 (Nakhamkin), U.S. Pat. No. 4,872,307 (Nakhamkin), U.S. Pat. No. 4,885,912 (Nakhamkin), U.S. Pat. No. 4,936,098 (Nakhamkin), U.S. Pat. No. 5,671,608 (Wiggs), U.S. Pat. No. 7,178,337 (Pflanz).
In view of the above, there is a continuing need for improvements in CAESR systems. What is desired therefore, are systems and methods which overcome at least some of the problems associated with prior art CAESR systems.